The invention relates to a device for evaporating and/or superheating a hydrocarbon.
It is known from GB-A-2 242 562, in order to increase the thermal efficiency of the heating of a methanol/water mixture, prior to a reforming reaction, for this mixture to be passed through two heat exchangers arranged in series. These heat exchangers are heated by the exhaust gases from in each case one catalytic burner. One of the heat exchangers is additionally heated by the exhaust gases which flow out of the upstream heat exchanger.
EP 0 184 814 B1 has disclosed a fuel evaporator in which a line which is wound helically around a central combustion chamber is provided for the medium which is to be evaporated. This coil containing the liquids which are to be evaporated is in direct contact with the flame of the burner, resulting in a high thermal load on the components of the evaporator.
DE 196 39 150 C2 has described a central heating device for a gas generation system. In this device, to provide thermal energy an operating medium is catalytically oxidized together with an oxygen-containing gas in a central component. The thermal energy which is generated in the central component is fed to further components of a gas generation system via heat transfer media. On account of the need for the thermal energy to be distributed by a heat-transfer medium to the individual components, corresponding line elements are required, connecting the central burner to the further components on the heat-transfer medium side. Considerable thermal losses may occur in these regions, which have an adverse effect on the overall efficiency of the installation.
EP 0 729 196 A, U.S. Pat. No. 4,670,359, EP 0 798 798 A and FR-A-1 553 361 each disclose systems in which a heat exchanger for heating or evaporating a medium is in each case heated by exhaust gases which are heated from a burner positioned upstream of the heat exchanger, either directly or with various intervening components, such as turbines or the like.
It is an object of the invention to provide a device for evaporating and/or superheating a hydrocarbon or a hydrocarbon/water mixture for a gas generation system of a fuel cell installation in which a heat exchanger is heated as efficiently as possible, which heat exchanger allows a high level of thermal efficiency and is able to react very quickly to changing conditions, such as load changes or the like, on the medium side.
This object is achieved by a device according to the present invention.
By combustion, thermal energy can be produced with a very high level of efficiency. If an exhaust gas from this combustion, which generally includes most of the thermal energy of the combustion, is then passed through the heat-transfer-side region of two heat exchangers, it is possible to provide thermal energy to the heat exchangers with a very high level of efficiency and at the same time to protect the heat exchanger from excessive thermal loading and from chemical loading being imposed on the heat exchanger material as a result of the flames from combustion acting directly on the material. Therefore, the device according to the invention enables a hydrocarbon or a hydrocarbon/water mixture to be evaporated with a very high level of efficiency.
Unlike installations which make use of a heat transfer medium, the device according to the invention has a very good dynamic response, since the thermal energy which is generated in the combustion can very rapidly be adapted to changing conditions. The fact that only the exhaust gas from the combustion flows through the heat exchanger means that time delays which have hitherto arisen as a result of the transfer of the thermal energy from combustion to a heat-transfer medium which then flows through the heat exchanger and on account of the much slower transfer of the thermal energy through the heat-transfer medium, which is generally in liquid form, are avoided.
Further advantages result from the structure of the heat exchanger which, on account of two fluids flowing through it, can be of relatively simple construction. On account of the loading on the material being lower than with direct heating with a flame or a catalytic burner, the heat exchanger can in this case be produced as a simple, for example soldered heat exchanger or evaporator, at low cost.
Moreover, there is no explosive gas mixture produced in the heat exchanger, since in this case only the exhaust gases from a combustion which has already taken place flow through the heat-transfer-side region of the heat exchanger. Therefore, the heat exchanger does not, for safety reasons, have to be designed to withstand these very high pressures which may arise in the event of an explosion, allowing further simplification with regard to production of the heat exchanger and, moreover, allowing the heat exchanger to be constructed with a very low mass. The low mass in turn produces the advantage that the heat exchanger, on account of its lower heat capacity, allows very rapid dynamic response to changing load demands on the fuel cell and therefore changing requirements with regard to the transfer of the thermal energy from the exhaust gas to the hydrocarbon or the hydrocarbon/water mixture.
The device according to the invention therefore offers the advantages of high efficiency with the simultaneous possibility of reacting very rapidly to changing requirements, for example a change in the power demanded from a fuel cell of the fuel cell installation.
By designing the two heat exchangers with different masses and the associated different heat capacities, it is particularly advantageously possible to achieve improved dynamics of the device according to the invention. These improved dynamics are associated with very good cold-start properties of the installation. According to the invention, this results from the mass and therefore the heat capacity of the first heat exchanger being significantly lower than the mass of the second heat exchanger. Consequently, the first heat exchanger can be heated better and more quickly and therefore has a better dynamic response. Naturally, the first heat exchanger, which is generally operated as an evaporator, can therefore also only transfer a lower evaporation capacity, but this is compensated for by the use of the second heat exchanger, so that the overall system remains able to provide a high evaporation capacity which is required.